Process for preparation of uranium metal



United States Patent i 3,140,171 PROCESS FOR PREPARATION OF URANIUM METAL John H. Trapp, Cincinnati, Ohio, assignor to the United States of America as represented by the United States Atomic Energy Commission No Drawing. Filed July 25, 1962, Ser. No. 212,492 8 Claims. (Cl. 75-841) My invention relates to the production of uranium metal and more particularly to the process wherein uranium tetrafluo-ride is reduced to metal with magnesium.

, Uranium metal is currently produced on a large scale by reduction of UP, with magnesium in a thermite-type reaction. In this process finely divided UR, and a slight stoichiometric excess of magnesium are blended, and the resulting mixture is compacted in a metallic reactor, com monly referred to as a reduction bomb, lined with the reaction by-product magnesium fluoride. The charged reduction bomb is then heated in a furnace to initiate the exothermic reduction reaction. Uranium metal formed in this reaction separates from the lighter by-product slag and collects as a regulus or derby at the bottom of the reactor. After cooling, the reactor is opened, and the uranium regulus is recovered. Adhering slag is then removed from the regulus by mechanical chipping, followed by chemical pickling with an acid solution. Further details of speific embodiments of this process may be seen by reference to Uranium Production Technology by C. D. Harrington and A. E. Ruehle, pp. 245293 (1959).

One of the problems presented in this process is control of the furnace temperature in heating the charged bomb to obtain maximum metal yield and quality. Furnace temperatures of about 1100 F. to 1400 F. have been employed, and the particular temperature used for a given operation, e.g., 1320 F. for charges producing 330 pounds uranium, has been selected to provide optimum yield and quality. The firing time required to initiate the reaction decreases with increasing furnace temperatures and higher temperatures Woud be advantageous in allowing a higher production rate in this step. At higher temperatures, however, the metal yield is decreased. Above the maximum temperature, that is, 1400 F., both the yield and quality of the metal decrease sharply, probably because of a pre-reaction wherein UF is partially reduced to UF instead of to metal. The heat of this pre-reaetion is dissipated before the metalproducing reaction occurs, and the temperature of the reaction mass is not sufficiently high to maintain fluidity long enough to allow adequate separation of the metal and slag. It is desired to eliminate this effect so that higher firing temperatures and, consequently, shorter firing times, may be employed without decreasing the yield or quality of the metal.

The uranium yield at the previous optimum firing temperature, which requires an extended firing time, has been about 97 percent. Even a slight increase in yield would result in substantial economic gain in large-scale production. The quality of the uranium regulus with respect to slag inclusions, surface roughness and impurity content has frequently been low, and improvement in this regard is also needed. Low quality of the uranium regulus ice results in a low yield in the subsequent melting and casting process and a high reject rate in the fabricated metal product.

It is, therefore, an object of my invention to provide a method of improving the reduction of UR, to metal with magnesium.

Another object is to provide a method of increasing the firing temperature in the initiation of said reduction process without adversely afifecting metal yield and quality.

Another object is to provide a method of decreasing the firing time in the initiation of said reduction process.

Another object is to provide a method of increasing the uranium metal yield and quality in said process.

Other objects and advantages of my invention will be apparent from the following detailed description and claims appended hereto.

In accordance with my invention the process for preparing uranium metal which comprises charging a refractory-lined reduction bomb with an intimate mixture of UP, and magnesium and heating the charged reactor in a furnace zone maintained at an elevated temperature until the mixture reacts, whereby uranium metal and slag are formed, is improved by providing a small mass of metallic lithium in the interior of said mixture and maintaining the heating zone at a temperature above 1400" F. The firing time required for initiation of the reaction is substantially decreased by these measures; the uranium metal yield is increased; and the quality of the uranium regulus is improved.

I have found that lithium additive in the reaction mixture allows the use of high firing temperatures which were previously unsuitable for initiation of this reaction. Although myinvention is not to be understood as limited to a particular theory, it is postulated that the UF -producing pre-reaction previously encountered at these temperatures is eliminated because the metal-producing reduction reaction is initiated by lithium before the pre reaction has time to occur. It is believed that the metalproducing reaction isinitiated as a result of melting of the lithium, the molten lithium providing intimate solid-liquid contact between the reactants. Once initiated the reaction propagates rapidly because of its exothermic nature. The reaction is initiated in the interior of the charge rather than at the heated edge, thus providing a better distribution of reaction heat and enhancing slag-metal separation.

The effect of lithium in combination with the higher firing temperature is in no way apparent from the previous use of lithium in the reduction of UR, with calcium. In the calcium system, lithium is added to obtain irnproved metal-slag separation by decreasing the melting ration, while in the present magnesium system only a small mass of lithium, e.g., 35 grams, 'and not enough to appreciably affect slag properties, is required. In addition, the calcium system with added lithium requires a heat-supplying agent such as iodine to initiate the reaction, while no additive other than lithium is employed in the magnesium system, this function being performed by the lithium itself.

A firing temperature of at least 1400 F., is critical to the attainment of significant advantages with the lithium additive in the magnesium system. At lower temperatures the firing time is decreased, e.g., about 30 percent at 1250" F., but metal yield and quality are not significantly changed. Temperatures up to about 1700 F. may be employed, but maximum advantage is obtained at about 1400 F. to 1500 F., and a temperature in this range is accordingly preferred. At this temperature the firing time is decreased to about 40 percent of that required under the conditions previously preferred for typical lots of material. Metal yield is increased about 1 to,2- percent and the quality of the regulus with respect to slag inclusions and surface roughness is substantially improved.

Lithium metal is provided in an amount of at least 25 grams, and preferably about 35 grams. No additional benefit is obtained with larger amounts. The lithium is amassed at one position within the reactant mixture, since this additive is ineffective in a dispersed state. Although not critical, the lithium may be conveniently, applied in the form of a single lump or small pieces or shot, e.g., about inch in diameter, placed together. Finely divided material presents a disadvantage in its difiiculty of handling.

The lithium mass is positioned within the charge to provide initiation at the desired locus. Placement of the lithium at the edge of the charge is ineffective since the reaction is initiated before the interior of the charge is heated to the extent required for slag-metal separation. The optimum position for the lithium may be varied with the size and geometry of the charge and the firing temperature used. For small, experimental-scale reactors, e.g., 7 inches in diameter,.it is preferred to place the lithium in an axial position slightly below the center of the charge. For larger reactors such as the 15-inch-diameter bomb currently used for large-scale production, place ment of the lithium in an intermediate position about one-half the distance between the center and diametral edge of the charge is preferred. An intermediate position is also preferred for the 42-inch-diameter dingot process reactor, wherein the exact location will depend on the number and location of internal heaters in the charge.

The method of my invention is not limited to a reactant charge of a particular size. This method, however, is particularly applicable to the process wherein a 330-pound uranium-content charge is reacted in a slightly tapered cylindricalvessel 15 inches in diameter and 42 inches high, this process being commonly referred to as the derby process. In this process the reaction charge, which consists of a finely divided mixture of UF and magnesium, is compacted to form a continuous mass in a metallic container lined with a refractory material, normally magnesium fluoride,and the container is capped and placed in a furnace for firing. The lithium additive may also be employed for the 3300-pound uranium charge in the dingot process. In this process furnace firing is supplemented by heating elements placed within the charge.

Although not critical to my invention, it is preferred to employthe lithium additive and increased firing temperature in combination with reaction conditions previously employed for optmum results. For example, a stoichiometric excess of magnesium of about 1 to 6 percent is preferred for maximum yield. UF of a particle size within the range of 1 micron to 200 microns and a tap density of 3.8 to 4.2 grams per cubic centimeter is also preferred, as well as a uranyl fluoride content of 0.5 to 2.5 weight percent in the UF The apparatus employed for the method of my invention is critical only to the extentthat'the reactant charge is contained in a refractory-lined metallic reactor. The preferred firing temperature, that is, 1400 F. to 1500 F., is applicable to metallic reactors lined with magnesium fluoride at a thickness of about to 1 /2 inches or other refractory material at a thickness such as to provide an equivalent thermal gradient across the liner. For example, fused dolomietic oxide (CaO-MgO) which was previously used as a liner material, has only about one-half the thermal conductivity of magnesium fluoride so that onehalf the thickness is equivalent for this material. For thicker or less conductive liners the firing temperature may be increased to obtain equivalent results. The conductivity of the metallic container, is so high in comparison to the refractory linerfthat itdoes 'not'significantly affect the temperature gradient. Although not critical, acarbon steel about /2 inch thick is preferred for the container. Other metals or alloys may also be employed. For heating the charged reactor a furnace capable of maintaining a constantt'e'mperature of at least 1400f F. is required. A resistance-heated electrical furnace is preferred for this purpose.

After the reduction reaction is effected, as evidenced by a large temperature rise in the reactor, the uraniumcontaining reactor may be handled by previously known methods to recover the uranium regulus. The reactor is preferably allowed to cool to a skin temperature of about 200 F. and is then quenched in water. The cooled reactor is opened, and the uranium regulus and slag are removed. Adhering slag is removed by mechanical chipping and/or chemical pickling. The uranium may then be fabricated by melting and casting to form ingots and working the ingots. V

My invention is further illustrated by the following specific example.

EXAMPLE A series of small-scale reduction experiments were conducted to determine the effect of added lithium and increased firing temperature on metal yield and quality. In each experiment a steel reactor 7 inches in diameter and 19 inches high was lined by jolt-packing with magnesiurn fluoride at a thickness of one-half inch. Thereact'or was then charged wtih 46.44 pounds of a blend. of UF and magnesium at a 4 percent stoichiometric excess. The composition and properties of the UF in weight percent, were as follows: UF 96.4; UO F 1.49; ammonium oxalate insoluble (uranium oxides), 2.02; H O,- 0.0l; tap density, 3.85 gm./cm. and particle size, lmicron to 200 microns. The blend was packed in the reactor by hand tamping, and the char'ged reactor was capped with magnesium fluoride and sealed with a steel cover. The sealed reactor was then placed-in a resistanceheated electric furnace and the furnace was maintained at a temperature of either 1250 F., 1400 F., or 1500 F. until the reduction reaction was initiated. The reactor was then removed from the furnace and allowed to cool in air. The cooled reactor was opened and the uranium regulus was jolted loose. Adhering slag was then removed to the extent possible by chipping with a hammer. The metal yield was calculated from the weightof the regulus. In each case the quality of the regulus was des g nated l, 2, or 3, 1 representing the least extent of slag inclusionand surfaceroughness and 3 representing the worst value for these factors. v particularly for 3, or very rough reguluses, removal of. all the slag by chipping was not possibleso that the yleld figures for these derbies are unduly high owing to the weight of the included slag. In some cases lithium at a level of 1 or 2 weight percent of the magnesium, 35 to grams, respectively, was added in the form of metal shot placed near the center (7 inches above the bottom line ,0! near the bottom (2 inches above the bottom In some cases,

liner). Further details and the results obtained may be seen by reference to the following table.

Table 1 EFFECT OF LITHIUM AND FIRING TEMPERATURE ON REDUCTION OF UFl Fur- Re- Firing Metal Reg nace duction Lithium Amount Time Yield ulus Tem- Number and Location (min- (per- Quality per utes) cent) ature Center35 grams. 80 106. 5 3 1, 250

Bottom35 grams 83 98. 9 2 1, 250

Bottom-70 grains..- 98 97. 3 2 1, 250

None 100 91. 0 3 1, 250

Bottom70 grams"..- 119 99.3 3 1, 000

1 Reduction was incomplete; the slag contained large amounts of UFs.

2 N0 regulus.

3 Bomb was heated twice, once for 45 minutes and a second time for 105 minutes.

The above results are further summarized in Table H below.

bomb with an intimate mixture of UP, and magnesium and heating the resulting charged bomb in a zone maintained at an elevated temperature until said mixture undergoes an exothermic reaction, whereby uranium metal and slag are formed, and recovering the resulting uranium metal, the improvement which comprises providing a small amount of metallic lithium in an amassed state within the interior of said mixture and maintaining said zone at a temperature of at least 1400 F.

2. The improvement of claim 1 wherein said lithium is provided in an amount of at least about 25 grams.

3. The improvement of claim 1 wherein said zone is maintained at a temperature of 1400 F. to 1500" F.

'4. The process for preparing uranium metal which comprises intimately mixing finely divided UF and a slight stoichiometric excess of magnesium, compacting the resulting mixture in a refractory-lined metallic reduction bomb, providing at least about 25 grams lithium metal in an amassed state within the interior of said mixture, heating the resulting charged reactor in a zone maintained at a temperature of at least 1400 F. until said mixture reacts, whereby uranium metal and slag are formed, and recovering the resulting uranium metal.

5. The method of claim 4 wherein said refractory is magnesium fluoride at a thickness of about to 1 /2 inches.

6. The method of claim 4 wherein said bomb is a substantially cylindrical vessel 7 to 15 inches in diameter.

7. The method of claim 4 wherein said zone is maintained at a temperature of 1400 F. to 1500" F.

8. In the process for preparing uranium metal which comprises charging a refractory-lined metallic reduction Table 11 SUMMARY OF REDUCTION RESULTS Furnace Temperature 1250* F. 1400" F. 1500 F.

Signifi- Signifi- Signifi- Control cant cant cant (no Experi- Difier- Control Experi- Dlfiter- Control Experi- Dinerlithium) mental ence mental ence mental ence (95% Con- (95% C on- (95% Confidence) fidence) fidence) Average Crude Yield (pereent). 97. 3 101.1 No 94. 2 97. 3 Yes 79. 8 98. 5 Yes. Average Firing Time (minutes) 113.0 80. 0 Yes-.- 92. 0 70.0 Yes 70.0 Average Regulus Quality- 2. 8 2. 7 No 2. 7 1. 0 Yes. 2. 8 1.0 Yes.

It may be seen from the above that the lithium additive in combination with a firing temperature of 1400 F. or 1500 F. results in higher yield, better metal quality and substantially shorter firing time than is the case without lithium at the previous optimum temperature (1250 F.) for this size batch. The lithium additive in combination with a 1250 F. firing temperature slightly shortened the firing time, but did not significantly altect metal yield or quality.

The above example is merely illustrative and is not to be understood as limiting the scope of my invention, which is limited only as indicated by the appended clams. It is also to be understood that variations in apparatus and procedure may be employed by one skilled in the art without departing from the scope of my invention.

Having thus described my invention, I claim:

1. In the process for preparing uranium metal which comprises charging a refractory-lined metallic reduction References Cited in the file of this patent UNITED STATES PATENTS 2,795,499 Peterson June 11, 1957 2,894,832 Magel July 14, 1959 2,977,220 Wood Mar. 28, 1961 FOREIGN PATENTS 617,076 Canada Mar. 26, 1961 

1. IN THE PROCESS FOR PREPARING URANIUM METAL WHICH COMPRISES CHARGING A REFRACTORY-LINED METALLIC REDUCTION BOMB WITH AN INTIMATE MIXTURE OF UF4 AND MAGNESIUM AND HEATING THE RESULTING CHARGED BOMB IN A ZONE MAINTAINED AT AN ELEVATED TEMPERATURE UNTIL SAID MIXTURE UNDERGOES AN EXOTHERMIC REACTION, WHEREBY URANIUM METAL AND SLAG ARE FORMED, AND RECOVERING THE RESULTING URANIUM METAL, THE IMPROVEMENT WHICH COMPRISES PROVIDING A SMALL AMOUNT OF METALLIC LITHIUM IN AN AMASSED STATE WITHIN THE INTERIOR OF SAID MIXTURE AND MAINTIANING SAID ZONE AT A TEMPERATURE OF AT LEAST 1400F. 